Jun 03, 2013

As ice sheets melted during the deglaciation of the last ice age and global oceans warmed, oceanic oxygen levels decreased and "denitrification" accelerated by 30 to 120 percent, a new international study shows, creating oxygen-poor marine regions and throwing the oceanic nitrogen cycle off balance.

By the end of the deglaciation, however, the oceans had adjusted to their new warmer state and the nitrogen cycle had stabilized – though it took several millennia. Recent increases in global warming, thought to be caused by human activities, are raising concerns that denitrification may adversely affect marine environments over the next few hundred years, with potentially significant effects on ocean food webs.

Results of the study have been published this week in the journal Nature Geoscience. It was supported by the National Science Foundation.

"The warming that occurred during deglaciation some 20,000 to 10,000 years ago led to a reduction of oxygen gas dissolved in sea water and more denitrification, or removal of nitrogen nutrients from the ocean," explained Andreas Schmittner, an Oregon State University oceanographer and author on the Nature Geoscience paper. "Since nitrogen nutrients are needed by algae to grow, this affects phytoplankton growth and productivity, and may also affect atmospheric carbon dioxide concentrations."

"This study shows just what happened in the past, and suggests that decreases in oceanic oxygen that will likely take place under future global warming scenarios could mean more denitrification and fewer nutrients available for phytoplankton," Schmittner added.

In their study, the scientists analyzed more than 2,300 seafloor core samples, and created 76 time series of nitrogen isotopes in those sediments spanning the past 30,000 years. They discovered that during the last glacial maximum, the Earth's nitrogen cycle was at a near steady state. In other words, the amount of nitrogen nutrients added to the oceans – known as nitrogen fixation – was sufficient to compensate for the amount lost by denitrification.

A lack of nitrogen can essentially starve a marine ecosystem by not providing enough nutrients. Conversely, too much nitrogen can create an excess of plant growth that eventually decays and uses up the oxygen dissolved in sea water, suffocating fish and other marine organisms.

Following the period of enhanced denitrification and nitrogen loss during deglaciation, the world's oceans slowly moved back toward a state of near stabilization. But there are signs that recent rates of global warming may be pushing the nitrogen cycle out of balance.

"Measurements show that oxygen is already decreasing in the ocean," Schmittner said "The changes we saw during deglaciation of the last ice age happened over thousands of years. But current warming trends are happening at a much faster rate than in the past, which almost certainly will cause oceanic changes to occur more rapidly.

"It still may take decades, even centuries to unfold," he added.

Schmittner and Christopher Somes, a former graduate student in the OSU College of Earth, Ocean, and Atmospheric Sciences, developed a model of nitrogen isotope cycling in the ocean, and compared that with the nitrogen measurements from the seafloor sediments. Their sensitivity experiments with the model helped to interpret the complex patterns seen in the observations.

Related Stories

Surface waters in upwelling regions of the ocean are generally rich in nutrients. Scientists had thought that these areas would have low rates of nitrogen fixation because diazotrophs-microbes that convert nitrogen gas from ...

(Phys.org)—During an expedition to the South Pacific Ocean, scientists from the Max Planck Institute for Marine Microbiology in Bremen, along with their colleagues from the GEOMAR and Christian-Albrechts ...

(Phys.org) —A Kansas State University research team has found that despite humans increasing nitrogen production through industrialization, nitrogen availability in many ecosystems has remained steady for ...

Nitrogen fertilizers from farm fields often end up in aquatic ecosystems, resulting in water quality problems, such as toxic algae and underwater 'dead zones'. There are concerns that fertilizers used on lawns may also contribute ...

A study led by researcher Lisa Kellogg of William & Mary's Virginia Institute of Marine Science shows that a restored oyster reef can remove up to 10 times more nitrogen from Chesapeake Bay waters than an ...

The ocean is a large reservoir of dissolved organic molecules, and many of these molecules are stable against microbial utilization for hundreds to thousands of years. They contain a similar amount of carbon ...

The fires superimposed on the satellite image of southeastern Australia designated by red spots may be indicative of "planned burns" by the Victoria region. This map: http://www.depi.vic.gov.au/fire-and-emergencies/planned-burns/planned-burns-now-and-next-10-days found on the Department of the Environment and Primary Industries for the State of Victoria shows the burns th ...

NASA's Aqua satellite captured an image of Typhoon Maysak as it strengthened into a super typhoon on March 31, reaching Category 5 hurricane status on the Saffir-Simpson Wind Scale. The TRMM and GPM satellites, ...

An international research team, led by a Virginia Tech geoscientist, has revealed information about how continents were generated on Earth more than 2.5 billion years ago—and how those processes have continued ...

The 2010 eruption of the Icelandic volcano Eyjafjallajökull grounded thousands of air flights and spread ash over much of western Europe, yet it was puny compared to the eruption 200 years ago of Tambora, ...

User comments : 0

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.